System level implementation of a c-band downconverter for a nanosatellite receiver

Abstract

In recent years, there has been a growing market interest in CubeSat missions around the world, particularly in the commercial sector for various applications. These include high temporal and spatial resolution imaging, which consist of a number of CubeSats forming a constellation. Consequently, the bandwidth requirements and associated regulatory challenges for these applications increase, which can respectively limit and delay a mission. Much research has focused on increasing data throughput over the lifespan of an operational CubeSat in orbit. Low earth orbit (LEO) CubeSats have popularly been using data rates typically around 9.6 kbps for uplink and sub 3 Mbps for downlink applications. However, this cannot keep up with the growing demand of new technologies. The trend nowadays is achieving high speed communication links. Thus, commercial applications favour frequencies above S-band (2.4 – 2.450 GHz), which will support high data rate downlinks in the order of about 3 Mbps – 1.7 Gbps. The congestion in the popular very high frequency (VHF: 144 – 146 MHz), ultra-high frequency (UHF: 435 – 438 MHz) and S-band frequencies for radio communications, require a shift to higher frequencies where more bandwidth is available, and there is less interference from other spectrum users when receiving weak signals. This research implements a C-band (4 – 8 GHz) downconverter for a CubeSat based receiver system. The goal of this study is to look at the usage of higher frequency bands that address the need for systems, which require high data rate. The research began with the literature study from which a research gap was identified, then a simplistic design workflow was implemented, followed by testing and validation of the downconverter system for a CubeSat receiver operating from 5.650 – 5.670 GHz. A system level approach using commercial-off-the-shelf components (COTS) was used to reduce development time and cost. To do this, each subsystem was implemented and tested individually before system level integration. The following performance parameters were validated: an input C-band frequency from 5.650 – 5.670 GHz with a center frequency of 5.66 GHz, an overall conversion gain of 40.205 dB, a noise figure less than 2.3 dB, a phase locked loop (PLL) based local oscillator frequency from 4.385 – 4.405 GHz, an output power of 4.52 dBm, a spurious response of -81.96 dBc/Hz, an out-band phase noise of -111 dBc/Hz at 1 MHz carrier frequency offset from a 4.395 GHz PLL carrier frequency and the output L-band frequency from 1.250 – 1.270 GHz at a center frequency of 1.268 GHz.